Electric vehicle traction control system and method

ABSTRACT

A traction control system and method are provided for electric vehicles with at least one drive wheel powered by an electric drive motor to maintain optimum maximum traction while the vehicle is driven on the ground. The traction control system includes drive means capable of transmitting torque through a vehicle drive wheel and controllable to move the vehicle over a ground surface. A preferred drive means is an electric motor designed to move the vehicle at desired ground speeds in response to operator input. Operator input requests a desired speed, and the system determines drive wheel torque required to produce the desired speed and provides maximum current to produce maximum torque to drive the vehicle with optimum traction at the desired speed. The system uses constant feedback to find maximum current corresponding to torque required for an inputted speed request to automatically control traction in any electric powered vehicle.

PRIORITY CLAIM

This application claims priority from U.S. Provisional PatentApplication No. 61/568,993, filed 9 Dec. 2012, the disclosure of whichis fully incorporated herein.

TECHNICAL FIELD

The present invention relates generally to traction control systems andmethods of traction control and specifically to a system and method fortraction control in an electric powered vehicle, including an aircraftdriven on the ground by an electric powered drive wheel.

BACKGROUND OF THE INVENTION

Traction control systems help to prevent or limit a vehicle's wheelsfrom slipping during acceleration on different surfaces. Traction of avehicle is established as its wheels contact a surface so that when thewheels are rotated, usually by a driving force, the vehicle will bemoved along the surface in a desired direction. The combination of thecoefficient of friction and the force exerted by a wheel against thesurface produces traction. When the coefficient of friction of thesurface is less than the force exerted, the wheel will slip duringacceleration of the vehicle, adversely affecting accelerationperformance and driving stability. Slippage can occur as a result ofexcessive accelerative forces applied to vehicle wheels or inadequatewheel to surface friction that can be present with wet or icyconditions. Once the condition is recognized, a vehicle driver,particularly in an automobile or like vehicle, may try to controlslippage by reducing engine power or by applying the brakes, both ofwhich can reduce the speed at which a drive wheel is rotating. Thedriver may not be immediately aware that slippage is occurring, however,and may not be able to take corrective action as quickly as required.

Various traction control systems have been proposed for automobiles andlike vehicles to automatically adjust traction between the vehicle'sdrive wheels and the road or ground surface to minimize accelerationslip. These include, for example, systems that control traction usingbraking force adjustment, engine throttle control, and engine fuelsupply control. Other traction control systems for automotive use havealso been proposed. In U.S. Pat. No. 6,002,979 to Ishizu (Nissan), forexample, an automobile traction control system in combination with afuel supply system that adjusts driving torque delivered to each drivewheel by adjusting engine power is described. This system monitorsslipping of a drive wheel with respect to a target drive wheel speed andincludes engine control means that cooperates with a fuel supply systemto decrease engine power by decreasing fuel supplied in response to adetected slipping condition. This system is sensitive to preventingengine stall when the speed of the drive wheel is reduced to a targetdrive wheel speed. A plurality of sensors is employed to assist with theelectronic control of the Ishizu traction control system.

The traction control device disclosed in U.S. Pat. No. 6,007,454 byTakahira et al (Toyota) automatically detects slipping conditions ofeach of the pairs of wheels in a four wheel drive automobile bycomparing the mean rotational speed of the front or rear drive wheels toa threshold value. A brake system is electronically controlled to applybrakes to at least one of the pairs of front or rear wheels, therebyexecuting traction control according to a selected gear transmissionratio. When optional vehicle speed sensors are included in this systemand wheel rotation speeds are compared to vehicle speed, theautomobile's engine can be controlled to decrease rotational poweroutput. Neither of the systems described in the aforementioned patentswould effectively control traction in a vehicle that is powered solelyby an electric drive motor or other electric drive means

Traction control systems useful in hybrid and electric vehicles,primarily automobiles, have also been proposed. U.S. Pat. No. 5,450,324to Cikanek (Ford), for example, discloses a combined traction controland antiskid braking system operatively connected to an electrictraction motor and a hydraulic braking system. Present vehicleparameters are monitored by sensors, and a processor responsive to thesensors calculates vehicle parameters not directly measurable todetermine whether the vehicle state requires traction control or brakingcontrol. A control strategy based on the determined vehicle state isused by the processor to provide commands to a motor controller tocontrol operation of the electric traction motor by reducing motortorque if traction control is appropriate or, alternatively, to a brakecontroller if hydraulic or regenerative antiskid braking control isneeded. The main focus of the traction control and braking systemdisclosed by Cikanek is to maximize regenerated kinetic energy duringbraking and minimize kinetic energy loss due to wheel slip, primarily toovercome battery energy storage limitations. The use of an electricdrive motor or other electric drive means to control traction is notsuggested.

The traction control system described in U.S. Pat. No. 6,577,944 byDavis uses existing engine speed sensors to determine the occurrence anddegree of wheel slippage by comparing whether two successive enginespeed readings exceed a selected threshold and generates an automaticproportional corrective action from the vehicle's engine, brakingsystem, or both. This system is designed primarily for automotiveinternal combustion engines and/or electric motors, although somelimited non-automotive uses in which a drive unit applies torque to arotating component that must overcome resistance are suggested. Theseinclude a turbine rotated by an electric drive motor and a power boatwith an internal combustion engine-driven propeller or screw. It is notsuggested that speed measurements could be eliminated or that torqueoutput of a drive unit or motor in the absence of speed measurementscould be controlled to control traction or that traction could becontrolled in an aircraft drive wheel.

A traction control system and method that eliminates the use of anelectric vehicle's antilock braking system to perform traction controlis described by Young in U.S. Pat. No. 5,758,014. A power inverter orcontroller uses rotor speed to control torque output of the electricvehicle's motor and thereby control traction. An encoder coupled to therotor provides signals that indicate rotor speed. Alternatively, theencoder could be replaced by wheel sensors. An algorithm monitors therate of change of the rotor speed as an indication of vehicle speed andcompares the rate of change of rotor speed to a programmable thresholdvalue to determine wheel slippage. A torque command from a controllercontrols the amount of current applied to the motor and the torquegenerated by the motor. When a loss of traction is experienced, reducingtorque decreases the rotor speed and allows the vehicle's drive wheelsto regain traction. The commanded torque value is based on the vehicleaccelerator pedal position or cruise control torque. It is not suggestedthat the described speed measurements could be eliminated or thatelectrical parameters affecting current of an electric motor could beused to directly control traction by maintaining a maximum supply ofcurrent in a response to a requested or commanded speed.

A need exists, therefore, for a traction control system and method ofcontrolling traction in an electric vehicle that relies on directcontrol of the current supply to an electric motor driving the vehiclewheels to maintain a maximum current supply for a vehicle operatorcommanded speed and control traction.

SUMMARY OF THE INVENTION

It is a primary object of the present invention, therefore, to provide atraction control system and method that directly controls electricoperating parameters and current supply to an electric motor drivingvehicle wheels to control traction.

It is another object of the present invention to provide a tractioncontrol system and method that controls traction in an electric vehicleby providing a maximum current to an electric drive motor in response toa specific operator speed command.

It is an additional object of the present invention to provide atraction control system and method for electric vehicles that controlstraction by modulating a torque/speed relationship to modulate vehiclewheel speed when the wheel is slipping.

It is a further object of the present invention to provide a tractioncontrol system and method that can be effectively employed toautomatically control traction in an electric vehicle under a wide rangeof ground surface and environmental conditions.

It is yet an another object of the present invention to provide atraction control system and method useful for controlling traction inany vehicle in which drive wheels are powered by electric drive motorsto move the vehicle on a ground surface.

It is yet a further object of the present invention to provide atraction control system and method for an aircraft equipped with drivewheels driven by electric drive motors to move the aircraft autonomouslyon the ground without reliance on aircraft engines.

It is a still further object of the present invention to provide aneffective traction control system and method that controls traction inelectric vehicles by providing a maximum current to an electric drivemeans in response to a specific operator command that can be employedalone or in combination with available traction control systems.

In accordance with the aforesaid objects, a traction control system andmethod are provided for electric vehicles with one or more drive wheelspowered by electric drive motors to maintain optimum maximum tractionwhile the vehicle is driven on the ground. The traction control systemincludes drive means capable of transmitting torque through a vehicledrive wheel and controllable to move the vehicle over a ground surface.The drive means is preferably an electric motor designed to move thevehicle at desired ground speeds in response to operator input. Operatorinput requests a desired speed, and the system determines drive wheeltorque required to produce the desired speed and provides maximumcurrent to produce maximum torque to drive the vehicle with optimumtraction at the desired speed. The system uses constant feedback to findmaximum current corresponding to torque required for an inputted speedrequest to automatically control traction in any electric poweredvehicle.

Other aspects of the present electric vehicle traction control systemand method and objects of the present invention will become apparentfrom the following description, drawings, and claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a torque/speed curve for an electric drive motorcontrollable to achieve optimum traction control according to thepresent invention;

FIG. 2 is a diagrammatic representation of an electric poweredautomobile wherein traction is controlled according to the presentinvention;

FIG. 3 a is a side view of an aircraft with landing gear wheel driveassemblies powered by electric drive means to move the aircraftindependently on the ground whereby traction is controlled according tothe present invention; and

FIG. 3 b is a schematic representation of a pair of aircraft drive wheelassemblies equipped with electric drive means mounted on the aircraft ofFIG. 3 a.

DESCRIPTION OF THE INVENTION

Traction control in any vehicle, whether powered by an electric motor oranother power source, requires modulating the rotational speed of thedrive wheels to match the vehicle travel speed over a ground surface.Wheel rotational speed is generally reduced by braking, for example,although other methods can also reduce speed, depending on the vehicle'spower source. Driving traction is produced by the force of the vehicle'swheels against the ground surface and the surface coefficient ofadhesion or friction, μ. The force of a wheel, which is directedperpendicularly toward the ground surface, may be described as includinga proportional force normal to the surface and a proportional forceparallel to the surface. The proportional force normal to the surface ispart of the traction frictional force. If a sufficiently large torque ordrive force is applied from the wheel to the ground surface tangent to acontact point between the wheel and the surface, the frictional grip, orstatic friction, between the wheel and the surface will be overcome.This causes the wheel to slip in relation to the surface so that thewheel is rotating at a higher surface speed than its forward travelvelocity, which creates a greater opportunity for slippage of the wheel.When a driving torque is applied to a vehicle tire through the wheel, atractive force, proportional to μ, develops at the area where the tiremounted on a wheel contacts the ground surface, known as the contactpatch. The tire tread in front of and within in the contact patch issubject to compression while the wheel is being driven. As a result, thedistance the tire and wheel travel when subjected to a driving torquewill be less than when the tire and wheel are free rolling. Unless theload is such that the wheel is restrained and/or corrected, the wheelwill continue to rotate at an increasingly faster speed, and continuedcontrol of vehicle speed or travel direction presents challenges.

A range of different kinds of electric drive motors can be used toprovide the power required to drive a wide variety of wheeled vehicles,including but not limited to, automobiles, trucks, trains, constructionvehicles, military vehicles, aircraft, drones, unmanned aerial vehicles,and the like, on different kinds of ground surfaces. The types ofelectric motors used for this purpose can include, for example withoutlimitation, toroidally-wound motors, axial flux motors, permanent magnetbrushless motors, synchronous motors, asynchronous motors, pancakemotors, switched reluctance motors, electric induction motors, or anyother electric motor geometry or type known in the art. The drive motorselected should be able to drive at least one vehicle wheel at a desiredspeed and torque. The present traction control system and method willalso function effectively with other numbers of drive wheels. Differentnumbers of drive wheels will be required for different applications, andthe number of drive wheels is not intended to be limited. One kind ofelectric drive motor preferred for this purpose is a high phase orderelectric motor of the kind described in, for example, U.S. Pat. Nos.6,657,334; 6,838,791; 7,116,019; and 7,469,858, all of which are ownedin common with the present invention. A geared motor, such as that shownand described in U.S. Pat. No. 7,469,858 for example, is designed toproduce the torque required to move a vehicle such as a commercial sizedaircraft at an optimum speed for ground movement. The disclosures of theaforementioned patents are incorporated herein by reference.

Other motor designs capable of the torque operation across a desiredspeed range required to move a specific vehicle on a ground travelsurface may also be suitable for use in the present invention. Onepreferred motor is a high phase order induction motor with a toptangential speed of about 15,000 linear feet per minute and a maximumrotor speed of about 7200 rpm, although motors with other tangential androtor speeds may be more suitable for moving other vehicles and are alsocontemplated to be within the scope of the present traction controlsystem. The aforesaid motor is particularly preferred for drivingaircraft drive wheels.

Both the electric motor selected to drive a particular vehicle and itsload may be described by their torque/speed curves and their inertia.Normal steady state operation occurs at the point where torque suppliedby the motor equals the torque consumed by the load. Any differencebetween torque supplied and torque consumed means that speed mustchange. The rate of speed change is determined by the torque differenceand the connected inertia. FIG. 1 illustrates an exemplary torque/speedcurve 10 for a high efficiency induction motor. It can be seen that thetorque/speed curve starts at a negative speed at 12, reaches a peak 14at break-away speed, drops roughly linearly to zero at synchronous speedat 16, and then falls asymptotically to zero again at 18. Such motorsproduce nominal full torque at about 50 revolutions per minute (RPM)below the synchronous speed, and the essentially linear output torque isthe difference between synchronous speed and actual speed. Peak torqueis generally 2 to 3 times the nominal operating torque; generallyregeneration peak torque is not equal to motoring peak torque. Thetorque/speed curve of FIG. 1 is a steady state approximation with itsown dynamics.

An electric motor torque/speed curve is directly controlled byelectrical operating parameters, including, for example, drive frequencyand drive voltage, preferably through an inverter in communication withthe motor. Drive frequency sets motor synchronous speed, and theinverter directly controls the drive frequency. The synchronous speed isthe speed at which the motor's torque falls through zero, as shown at 16in FIG. 1. Above the synchronous speed, the motor actually acts as agenerator and produces a negative or braking torque. This can introducean important failure mode, since extensive regeneration will cause DCrail voltage to increase, and inverter components can be damaged. Thedrive voltage sets motor magnetic flux density, which sets the magnitudeof the curve. An induction motor has a characteristic torque/speed curvethat stays relatively constant in overall shape. Setting the synchronousspeed sets where this curve crosses the speed axis, and the appliedvoltage acts as a scale factor on this curve. Magnetic flux density canadjust the torque scale, and synchronous speed slides the curve fromside to side.

Because a motor torque/speed curve is controlled by two parameters,generally any specific single value of torque and single value of speedis a member of a family of different torque/speed curves. As a result,operating voltage can be traded for slip. The difference between speedof the motor's rotor and synchronous speed is identified as rotor slipand is expressed as a fraction of synchronous speed or as rotor slipfrequency and expressed in Herz (Hz) or in radians per second. Normalmotor operation generally uses a rotor slip frequency of less than 2 to3 Hz, which indicates operation in the nearly linear region 20 of thetorque/speed curve of FIG. 1 between breakdown torque and synchronousspeed. Ideally, a torque/speed curve can be predicted, given differentdrive voltage and frequency inputs and motor state inputs, includingtemperature and the like. The traction control system and method of thepresent invention does not directly control the torque output of themotor; rather, the switching elements in an inverter that supply powerto the motor are controlled. Parameters controlled directly are voltage,actually pulse width modulation (PWM), drive frequency, and phasesequence. A motor model useful with the traction control system andmethod of the present invention will allow the prediction of atorque/speed curve, like that of FIG. 1, given different inputs of drivevoltage and frequency and inputs of motor state, such as, for example,temperature and the like.

In an example of traction control in a vehicle powered by an electricmotor, an open loop type of torque control can be used effectively.Vehicle operator input, adjusted as required by limit laws, temperaturelimits, wheel slip control, and the like, sets a motor commanded torquevalue, preferably based on a motor model as described above. Atorque/speed curve that optimizes efficiency to produce a motorcommanded torque at an expected rotor speed is selected. A particulartorque/speed curve will only produce a commanded torque if the rotorspeed is as expected. If the actual rotor speed is different from thatused in the calculations, the output torque will be different. Whenmotor torque is commanded, the following events occur. Given the presentspeed of the rotor, drive frequency and voltage that will produce arotor speed on the torque/speed curve at the desired torque arecalculated. Obtaining an accurate desired torque requires a good motormodel and an accurately obtained rotor speed. In the present tractioncontrol system and method, motor torque is not actually programmed;instead, the motor torque/speed curve is programmed.

Vehicle operator inputs can include, for example, motor commanded torque(from desired vehicle speed), rotor speed, motor stator temperature, andavailable inverter voltage. Rotor speed can be instantaneous or averagedand may be measured directly, such as with a resolver or the like, orpredicted using available sensorless techniques. Although acceptablefunction of the system is possible without valid temperature data,improved motor performance and torque accuracy are improved whentemperature correction is performed, preferably after directly measuringtemperature either a sensor or stator winding resistance. The availableinverter voltage is obtained by directly measuring DC rail voltage. Inthis open loop motor torque control system, updating of frequency andvoltage values should preferably occur between 5 and 10 Hz. Since theupdate rate provides a natural limit to wheel acceleration, it ispreferred that this is low to facilitate wheel loss of tractiondetection. Alternatively, open loop frequency and voltage values can beupdated at a high rate, but filtering the rotor speed to limit wheelacceleration is required. In the open loop situation described, aninduction motor is a very good approximation of a constant speed device.If, for example, a synchronous speed of 2000 rpm is programmed, themotor will naturally remain in the range of 2000 rpm±50 RPM over itsentire torque capability.

During a loss of traction, excess torque, applied by the motor to a geartrain to a wheel but not transferred through a contact patch will causethe wheel and gear box to accelerate. As the rotor, gear box, and wheelaccelerate, the output torque of the motor will naturally drop. Thetorque demanded by a slipping wheel is not zero, and at an equilibriumpoint where the wheel is spinning more rapidly than the vehicle ismoving, but slower than synchronous speed, the wheel is still capable ofproviding some traction. Because of the steepness of the torque/speedcurve at 20 near synchronous speed at 16 (FIG. 1), the differencebetween normal operating speed and this initial slipping speed willactually be quite small. For example, 50 rpm at the motor is 2.5 rpm atthe wheel so that in a vehicle with a wheel radius of about 11.5 inches,for example, a 2.5 rpm speed difference is a 900 ft/hr speed difference,which is not very much slip. Even at zero output speed, an inductionmotor still needs to have about 50 rpm of slip. Consequently, the motormagnetic field will be rotating at 50 rpm even as the motor's rotor isstationary. In the event of a loss of traction from when the vehicle isat a standstill, the motor will start spinning at something less than 50rpm. The exact speed will depend upon the commanded torque and theactual torque taken by the contact patch.

As discussed above, in an open loop control feedback system, duringnormal torque control operation, the changed speed of a vehicle wheelmust be detected, and this information will be supplied to a processorto be compared to a motor model torque/speed curve to calculate a newsynchronous speed to use to produce a desired torque. In a loss oftraction situation, the contact patch is already slipping and cannotsupport the desired torque, so the wheel accelerates more to the newsynchronous speed. If this process continues, the wheel will spin out ofcontrol. Sudden acceleration of a wheel can also provide a way to detectthe existence of wheel slip and allow a different control loop to takeover. The acceleration of a slipping wheel is likely to be substantiallygreater than the acceleration of that wheel if the wheel is driving avehicle. Friction associated with spinning up the wheel is very low, andthe net result is that during a loss of traction, the wheel speed willaccelerate in a step fashion to a new speed, with the acceleration stepfar more rapid than anything the vehicle itself could do.

Once this loss of traction is detected, a number of different approachescan be taken. For example, the vehicle operator could reduce therequested speed so the torque drops to zero, wait long enough for thewheels to stick to the surface, obtain a new measure of speed, and thenbring torque back up until traction is lost again. Alternatively, theslip as measured right after the acceleration event could be used toestimate the slipping torque that the wheel is applying through thecontact patch to the ground surface, and the motor model could be usedto put this torque at what is assumed to be the correct speed, whichshould cause the wheel to slow down and stick to the surface. Torquecould then be brought up again until traction is lost. Anotheralternative is to ignore that the wheel is slipping and use some othermeasure of vehicle speed to enter a speed control mode where synchronousspeed and ground speed are separated by a value that should give acommanded or target torque. Either the target torque is reached or thewheel slips slightly.

During a slip condition in a vehicle with an open loop torque controlsystem, a wheel may accelerate to just short of synchronous speed, andthen acceleration would stop, with no further acceleration until thetorque control system updates the synchronous speed. Large and sustainedacceleration is only possible if the synchronous speed is updated.Otherwise, acceleration pulses must be detected. If wheel slipconditions are not properly detected, an acceleration pulse will beproduced each time synchronous speed is updated. On detection of a wheelslip event, control of wheel slip is likely to take precedence over openloop torque control.

A method that could be used to detect if a wheel is slipping involvesdetecting sudden acceleration. Rather than look for a single suddenacceleration event, however, a vehicle operator should look for amodulated acceleration. For example, to control the motor, bothsynchronous speed and the “stiffness” of the motor torque/speed controlcurve are directly controlled. The stiffness of the torque/speedresponse is set by motor magnetic field strength, which is set by theratio of applied voltage to drive frequency. A wide range of synchronousspeed and stiffness will give the same output torque at the same targetspeed, which amounts to selecting the family of torque/speed curves thatall go through a single target operating point. Normally, a specifictorque/speed curve that produces optimal efficiency is selected.Switching between two or more torque/speed curves, ideally two curvesthat both show a reasonable, if not optimal, efficiency of differentcombinations of synchronous speed and torque stiffness, should give thesame torque at a target wheel speed. If a wheel is not slipping, and thespeed estimate and motor model are correct, then this modulation will bevirtually undetectable. Motor torque will remain constant, and speedshould remain constant. If the wheel is released from the groundsurface, however, the torque/speed curve modulation will be seen as amodulation of wheel speed.

An electric motor can be used to measure available vehicle traction in anumber of ways. Given a fixed torque/speed curve, motor output torquecan be calculated by knowing rotor speed. This is done by running thesame motor model calculations used for the open loop torque control, butin reverse. Since torque naturally drops to zero at a speed onlyslightly faster than a wheel not slipping speed, the wheel slippingtraction can be measured. Another approach is modulation of thetorque/speed curve. By adjusting both drive frequency and drive voltage,different torque/speed curves that all share the same target operatingpoint can be commanded. If the motor is actually operating at the targetoperating point, this modulation will have virtually no effect on wheeloperation. However, if the motor is not at the target operating point,but the wheel has traction, torque pulsations will be produced. Finally,if the wheel is slipping, then modulating the torque/speed curve willmodulate the wheel speed. A third approach is torque modulation. If thewheel is slipping, and the torque is reduced to zero, the wheel willquickly stop slipping. Torque can then be restored until the wheel slipsagain. Wheel slip can be probed by raising the torque above the point ofthe last slip event.

As indicated above, the traction control system of the present inventioncan be used effectively with a wide range of electric powered vehicles.The vehicles discussed below are intended only as exemplary of twopossible electric powered vehicle types with which this system can beused to maintain optimum traction while the vehicle is driven over aground surface. The use of the term “wheel” is also intended to includea tire mounted on a wheel. As noted above, the contact patch referred toherein describes the area of contact between a wheel and the groundsurface over which the wheel is traveling or a tire mounted on the wheeland the ground travel surface.

FIG. 2 is a diagrammatic illustration of an electric powered automobile30. The automobile has an electric drive motor 32 with a rotor 34. Aninverter 36 directs power to the drive motor 32 from a power supply 38,such as, for example, an array of electric batteries or the like. A reardrive wheel 40, preferably one of a pair of drive wheels, is driven byelectric power provided by the electric motor 32. Only one drive wheel40 of a pair of rear drive wheels is shown in FIG. 2. A pair of frontdrive wheels, including drive wheel 42, would be located at the front ofthe automobile in a front wheel drive vehicle. A wheel/ground surfacecontact patch 44 is shown between wheels 40 and 42 and a ground travelsurface 46. A traction control system controller 48 that preferablyincludes a processor, controller, and intelligent software, is providedto perform system traction control functions as described herein. Thetraction control system of the present invention can also be usedeffectively in a four wheel drive vehicle. At least one drive wheel isrequired for traction control in accordance with the present invention.Control of traction for any number of drive wheels can be achieved bythe present system and method.

In operation, the driver of the automobile selects a desired travelspeed, and software in the system controller 48 uses feedback tocontinuously find the maximum current draw from the inverter 36 to theelectric motor 32 to maintain a torque that will produce the desiredspeed. The traction control system can be set by continuously finding ahighest current consumption case, within a given range, and applyingthat identified maximum current. If the current decreases, that is anindication that one or more of the drive wheels are slipping. Thetraction control system will automatically cause the torque and, thus,the speed to be decreased until optimum traction is restored. Feedbackmechanisms, including those described above, can be used toautomatically maintain the current as close to 100% as possible,although if the current is set to less than 100%, wear on tires mountedon the drive wheels will be reduced. Maintaining the current at a levelin the range of about 95% to 98% of maximum current effectively reducestire wear and still maintains optimum traction.

FIG. 3 a illustrates, in side view, an aircraft 50 equipped with aground travel control system that includes a pair of drive wheelassemblies 52 with electric drive means 54, preferably electric motorsas described above, capable of translating torque through at least onedrive wheel 56 to drive the aircraft autonomously on a ground surface 55without reliance on the aircraft's engines. Although a pair of drivewheel assemblies is shown and described, the present traction controlsystem and method will operate effectively with one drive wheel assembly52. The drive wheel assemblies 52 are shown on the aircraft nose landinggear 58, which is the preferred location. One or more drive wheelassemblies could also be located on a main landing gear, such as at 60.Electric power for the electric motors 54 is directed, preferablythrough one or more inverters (not shown) associated with the motors 54from an aircraft power source 64, which is preferably the aircraftauxiliary power unit (APU). Aircraft ground travel speed controls areincluded in the array of controls (not shown) provided in the aircraftcockpit 62 so that an aircraft pilot can input a desired speed formoving the aircraft on the ground using the ground travel systemelectric motor or motors. A traction control system controller 66, whichis similar to controller 44 in FIG. 2, is provided in a convenientlocation relative to the motors 54 and preferably includes processor andcontroller elements as well as intelligent software capable ofcontrolling traction as described herein. The traction control system ofthe present invention automatically converts speed to torque andprovides the maximum electric current required to provide the requestedtorque to drive the aircraft's wheels to move the aircraft independentlyduring ground travel at the desired speed. When the pilot commands aspeed, the present traction control system software uses feedback, asdescribed herein, to continuously find the maximum current draw for thatspeed request, automatically optimizing traction. Although optimumtraction can be achieved without measuring or inferring wheel oraircraft speed, there are circumstances when such measurements may beneeded and can be obtained as described above.

During an aircraft drive wheel slip condition, there is a sudden changein the load/torque speed curve (FIG. 1) and a change in the connectedinertia. The torque load is likely to drop considerably because anaircraft wheel contact patch 68 cannot sustain significant shear force.Inertia is also likely to drop significantly, since the drive motor (54)will now be coupled only to a drive wheel (56) and not to the entireaircraft. When the wheel contact patch 68 slips, the wheel and motorwill accelerate at a rapid rate, and as the wheel accelerates, the motoraccelerates toward synchronous speed. Motor torque output will drop. Inaddition to the motor forces driving the wheels, several mechanical“springs” will act on the wheels. When the wheel contact patch 68 isreleased, the aircraft nose strut 70 will move backward toward the mainlanding gear 58, momentarily reducing the ground speed of one or more ofthe nose wheel drive assemblies 52. When the wheel contact patch 68releases, the wound “spring” of the wheel and a gear train that ispreferably associated with the motor will unwind, causing the wheel toaccelerate. When the wheel contact patch releases, the unwinding“spring” of the wheel will change the effective radius of the wheel.Assuming nothing changes, then, in the slipping condition, a newequilibrium condition will result. Although the wheel is slipping, therewill still be some friction at the contact patch 68, and the wheel loadwill still have a torque/speed curve (FIG. 1).

Another method for detecting a loss of traction in an aircraft drivewheel like a drive wheel 56 involves comparing the moment of inertia ofa drive wheel to the moment of inertia of the aircraft. If all the wheelmass is assumed to be concentrated in a ring with the radius of thecontact patch 68 formed between the wheel 56 and ground surface 55, thenthe ratio of the wheel inertia to aircraft inertia is the mass ratio.Additionally assuming that the wheel weighs 100 kg and half of theaircraft mass is 25,000 kg, then this is a 250:1 difference inacceleration rate at a given torque, ignoring friction. While this mayoverstate the wheel inertia, the acceleration of a slipping wheel may be100 to 500 times the acceleration of that wheel, if it is driving anaircraft. The friction associated with spinning up the wheel is verylow, mainly from ball bearings, while the friction for accelerating theaircraft is a large fraction of the available tractive effort. The netresult is that during a loss of traction, the wheel speed willaccelerate in a step fashion to a new speed, with the acceleration stepbeing far more rapid than any acceleration from the aircraft itself.When this loss of traction is detected, any of the approaches discussedabove could be used to restore or maximize traction.

Loss of traction on one or more aircraft nose gear 58 drive wheels 56will mean a loss of acceleration and will reduce nose wheel steering.Even a slipping spinning nose wheel will provide lateral force andturning moment. The weight on the main landing gear 60 and thedifferential braking ability of the main landing gear will provide someaircraft steering capability in the event the nose gear drive wheelshave completely lost traction.

Traction control can rely on the direct and indirect measurement ofwheel and vehicle speed, as well as inferred and computed true speed andacceleration. The present invention bases traction control on measuringcurrent provided to a drive motor powering a vehicle drive wheel againstspeed. The relationship between speed and torque is shown and describedin connection with FIG. 1. The current to be supplied to a drive motoror motors is compared with vehicle operator speed demands as describedin connection with FIGS. 2, 3 a and 3 b. In the case of an aircraft, apilot inputs a speed command or requests a desired aircraft groundtravel speed. As discussed above in connection with controlling electricautomobile traction, software uses feedback to continuously find themaximum current draw for that speed request. When more speed is demandedby a pilot or other vehicle operator, the current or power draw shouldincrease. When power draw does not increase, the system backs up thecommanded drive frequency to a peak and continues hunting high and lowto ensure that the commanded drive frequency remains near or below thatpeak. When the drive motors seeking to input a required torque to thewheels detect either a current drop or a flattening of the slope in thetorque/speed curve (FIG. 1), the system deducts that wheel slip isoccurring. The moment of maximum traction will be at or slightly belowthe peak current draw; when the wheels slip properly, power draw isreduced. This traction control system does not rely on speedmeasurements or other inputs or calculations to determine slip. If ahigher speed input does not require a higher power draw, the wheels areslipping. The operation of the system is the same whether the aircraftor other vehicle is traveling on a hill or whether there is a head ortail wind or some other environmental condition that could affectvehicle speed and/or traction.

A resolver could be used for motor position sensing, which can make lowspeed resolution almost trivial. A resolver useful in this instance isan analog device with only a few pulses per revolution that has theanalog capability to resolve fractional “pulse” position. A resolver todigital chip should be able to report both position and speed withbetween 10 and 16 bit resolution. This arrangement may be limited to thelowest resolution for speed reasons, which would produce about 1024counts per resolution at the motor and about 20 times this at the wheel.In an aircraft, if both nose drive wheels lose traction, based onmaximum motor torque and aircraft mass a reference rotation value shouldnot increase faster than about 0.5 m/sec². This value may change, suchas, for example, in a situation in which a light aircraft is travelingdownhill with a tailwind.

Loss of traction could also be detected by measuring control statesindicative of motor torque, such as, for example, the relationshipbetween AC current and current phase and/or the relationship between ACvoltage and frequency. A review of these values will detect a loss oftraction.

The traction control system of the present invention can effectivelymaintain traction during electric vehicle ground travel under adverseweather conditions. Preventing wheel slip and maintaining optimumtraction on reduced traction ground surfaces and/or during adverseenvironmental conditions has presented a particular challenge foravailable traction control systems. The present system maximizestraction control under a wide range of surface conditions where tractionis reduced. The system software detects the speed on a ground surfacethat will draw maximum power, as discussed above, to optimize traction.When the ground surface is covered with ice, for example, and thecurrent is maximized, maximum heat and pressure are transferred from oneor more wheel to the icy surface to melt ice under the wheels, allowingthe vehicle wheels to obtain better traction. As the ice melts, theground surface is exposed, and traction can be maximized. The vehiclecontrols could include a specific deicing mode setting that the operatorcould set manually to draw maximum current and activate deicing actionwhen a ground travel surface, whether a road, runway, rails, or thelike, is ice covered and likely to cause a loss of traction.

Another variation of the present traction control system is programmedto take into account known or inferred losses from heating between themotor and the contact patch where the wheel meets the ground. This couldinclude, for example, heat losses from electronics, wires, and gears aswell as the motor. Aiming for a peak current draw that is less than100%, such as on the order of 98%, for example, might be moreappropriate in this situation. The peak current draw could also beconsidered to occur when a drop off in a still-climbing torque/speedcurve (FIG. 1) is detected.

An additional approach to traction control is provided by the presentinvention. This approach is intended primarily for aircraft withelectric drive-controlled ground travel systems, but could also beadapted for other vehicles. Instead of building traction control intothe aircraft or vehicle drive system, a control device commonly used tocontrol the drive system, such as, for example, a joystick in anaircraft cockpit, could be adapted to incorporate control features of atraction control system and would be controllable to interact with thedrive system to control traction. The joystick preferably is designed tobe a force feedback joystick. The position of the joystick willcommunicate a desired motor torque to the drive system. The forcefeedback is set by the torque that the motors are actually producing. Inthe event of a loss of traction, the motors will accelerate untilacceleration limit laws programmed into the system are reached, when theoutput torque will decrease. The pilot or other vehicle operator willfeel the drop in torque through the joystick and can respond manually torestore traction to an optimum state.

The electric vehicle traction control system and method of the presentinvention has been described with respect to preferred embodiments. Thisis not intended to be limiting, and other arrangements and structuresthat perform the required functions are contemplated to be within thescope of the present invention.

INDUSTRIAL APPLICABILITY

The electric vehicle traction control system and method of the presentinvention will find their primary applicability in maintaining optimumtraction between wheels and ground surfaces in a wide range of vehiclesthat employ electric powered drive means to move the vehicles over thesesurfaces. The present traction control system and method can be appliedto maintain optimum traction under adverse and other environmental andground conditions in electric powered vehicles including automobiles andlike vehicles, light and heavy construction vehicles, military vehicles,aircraft, drones, unmanned aerial vehicles, and trains.

The invention claimed is:
 1. A system for controlling and maintainingoptimum traction between wheels of a vehicle and a surface on which saidvehicle is driven, wherein said vehicle is equipped with one or moredrive wheels electrically powered to drive said vehicle, said tractioncontrol system comprising: (a) electric drive means designed totranslate torque through said one or more drive wheels to drive saidvehicle on said surface; (b) a vehicle operator input actuatable toinput a desired vehicle speed into said traction control system; (c) aprocessor adapted to receive the desired vehicle speed and automaticallydetermine a maximum electric current to be supplied to said electricdrive means to translate torque required to drive the vehicle at thedesired speed and optimum traction to said one or more drive wheels; (d)a controller adapted to automatically control, in response to saidprocessor maximum electric current determination, a supply of electriccurrent to said electric drive means corresponding to said maximumelectric current and maintain said supply of electric current to drivesaid vehicle at said desired speed and optimum traction until saidvehicle operator input is actuated to input a different desired speedinto said system; and (e) vehicle controls comprising an automaticallyor manually controllable deicing mode activatable to generate sufficientheat and pressure between said one or more drive wheels and said surfaceto melt ice on said surface, thereby improving traction, when saidcontroller determines that less than a maximum supply of power indicatesa loss of traction and said ground surface is ice covered.
 2. Thetraction control system of claim 1, wherein said vehicle is an electricpowered vehicle selected from the group comprising automobiles, trucks,construction vehicles, military vehicles, aircraft, and trains.
 3. Thetraction control system of claim 2, wherein said vehicle is anautomobile.
 4. The traction control system of claim 2, wherein saidvehicle is an aircraft.
 5. The traction control system of claim 1,wherein said electric drive means comprises an electric motor selectedfrom the group comprising toroidally-wound motors, axial flux motors,permanent magnet brushless motors, synchronous motors, asynchronousmotors, pancake motors, switched reluctance motors, and electricinduction motors.
 6. The traction control system of claim 1, whereinsaid processor comprises software adapted to automatically intelligentlydetermining a torque/speed curve controlled by electrical operatingparameters for said electric drive means and the torque translated tosaid one or more drive wheels depends on the desired vehicle speedinputted into said system.
 7. A system for controlling and maintainingoptimum traction between wheels of a vehicle equipped with one or moredrive wheels electrically powered to drive said vehicle and a surface onwhich said vehicle is driven, said traction control system comprising:(a) one or more electric drive motors controllable to translate torquethrough said one or more drive wheels to drive said vehicle on saidsurface in communication with a source of electric power through one ormore inverters; (b) a vehicle operator input in communication withswitching elements in said one or more inverters comprising at leastmotor commanded torque from desired vehicle speed, rotor speed, motorstator temperature, and available inverter voltage; (c) a processoradapted to receive a selected vehicle operator input and toautomatically determine a maximum electric current to be supplied tosaid one or more electric drive motors to translate the torque requiredto drive the vehicle at a desired speed and optimum traction for surfaceconditions to said one or more drive wheels, said processor comprisessoftware adapted to program and select a motor torque/speed curve forthe said one or more electric drive motors that optimizes efficiency toproduce the motor commanded torque at an expected rotor speed and to setthe motor commanded torque at the expected rotor speed; and (d) acontroller adapted to control automatically, a supply of electriccurrent from said source of electric power to said one or more inverterscorresponding to said maximum electric current to maintain the motorcommanded torque at the expected rotor speed in accordance with theselected motor torque/speed curve and drive said vehicle at said desiredspeed and optimum traction.
 8. The traction control system of claim 7,wherein said vehicle is an electric powered vehicle selected from thegroup comprising automobiles, trucks, construction vehicles, militaryvehicles, aircraft, and trains.
 9. The traction control system of claim8, wherein said vehicle comprises an aircraft, said one or more electricdrive motors comprise high phrase order electric induction motors, andsaid motor torque/speed curve has a constant shape.